Regeneration of anion exchange resins

ABSTRACT

The invention relates to a method for converting strong-base anion exchange resins from the monovalent anion form to the hydroxide form. A . .solution.!. .Iadd.source .Iaddend.of polyvalent anions is first passed through the resin to displace the monovalent anions. A solution of alkali metal hydroxide is then passed through the resin to convert it to the hydroxide form, and to produce an effluent solution of hydroxide ions and polyvalent anions. This effluent solution is then neutralized with an acid containing .Iadd.a source of .Iaddend.polyvalent anions, and the neutralized solution is passed through a second batch of resin in the monovalent anion form. In the preferred embodiment, a portion of the effluent is retained without neutralization and is delivered to the second batch of resin after the neutralized effluent to perform a portion of the hydroxide regeneration.

The present invention relates to an improved method for convertingstrong-base anion exchange resin having quaternary ammonium activegroups from the monovalent anion form to the hydroxide form.

Strong base anion exchange resins having quaternary ammonium activegroups are well known in the art. Such resins and the method for makingthem are described in U.S. Pats. Nos. 2,614,099 and 2,591,573. Suchresins are prepared by reacting a tertiary amine with an insolublecross-linked copolymer of a monovinyl aromatic hydrocarbon and a divinylaromatic hydrocarbon such as a copolymer of styrene and divinylbenzene,or a copolymer of styrene, ethylvinylbenzene and divinylbenzene. Such apolymer contains halomethyl groups on its aromatic nuclei. Other anionexchange resins of the strongly basic quaternary ammonium type aredescribed in U.S. Pats. 2,597,440 and 2,597,494. Such resins comprisethe reaction product of a tertiary amine and a haloalkylated insolublecopolymer of one or more vinyl aromatic compounds such as styrene orvinylanisole and a minor proportion of a polyolefinic compound such asdivinylbenzene, isoprene, butadiene, and trivinylbenzene with oneanother to form a product containing quaternary ammonium substitutegroups on the aromatic nuclei. The strongly basic quaternary ammoniumanion exchange resins are prepared by haloalkylating, preferablychloromethylating, an insoluble vinyl aromatic resin and reacting thehaloalkylated product with a tertiary amine such as triethyl amine,trimethyl amine, dimethylbenzyl amine, dimethylethanol amine, ordimethyl aniline. Other tertiary amines such as tributyl amine,N-methylmorphonine and pyridine are also useful, but the products aresomewhat less stable than those made with the tertiary amines mentionedearlier.

The strongly basic quaternary ammonium amines exchange resins in theirhydroxide form are extremely strong bases which neutralize acids, splitsalts, and exchange ions in a neutral to alkaline aqueous solution.Thus, when a solution of sodium chloride is passed through a columncontaining a quaternary ammonium anion exchange resin to its hydroxideform, the chloride ions of the salt solution are exchanged for thehydroxyl groups on the resin and the liquid leaves the column as asolution of sodium hydroxide.

In use, strong-base anion exchange resins of the type described arefrequently exhausted by conversion from the hydroxide form to themonovalent anion form, e.g., the chloride, nitrate, bromide, etc., form.It is frequently desired to regenerate these resins to the hydroxideform so that they can be re-used.

In other applications, for example the condensate polishing processdescribed in U.S. Pat. No. 3,250,703 to Levendusky, which is assigned tothe assignee of this application, it is not ordinarily desired toregenerate the resin. Instead, this resin is supplied to filter elementsas a precoat layer, and is discarded when exhausted. In suchapplications, however, the resin when initially employed to form aprecoat layer is preferably in a very high state of regeneration. Sincefresh strong-base quaternary ammonium-type anion exchange resins aregenerally supplied by the manufacturer in the chloride form, it isnecessary to convert these resins to the hydroxide form prior to use.

It is known that anion exchange resins of the strongbase type havingquaternary ammonium groups may be most efficiently regenerated from themonovalent anion (e.g. chloride) form to the hydroxide form by firstconverting the resin to a polyvalent anion form (including divalentforms), and then converting the resin to the hydroxide form. Such amethod is described in U.S. Pat. No. 2,723,245.

In the context of the present invention, it will be understood that theterm "monovalent anion form" refers to resin forms other than thehydroxide form. Examples of such monovalent anions include chloride,bromide, and nitrate. These monovalent anions are exchanged forhydroxide anions when the resin is exhausted, or occupy the ion exchangesites of fresh resin as supplied by the manufacturer.

Generally, the present invention relates to an improved method forconverting strong-base anion exchange resin having quaternary ammoniumactive groups from the monovalent anion form to the hydroxide form. Incarrying out the method, a . .solution.!. .Iadd.source .Iaddend.ofpolyvalent anions (i.e., anions having a negative charge of two or more)is passed through a first batch of resin to displace the monovalentanions . .with the polyvalent anions.!.. Subsequently, a solution ofalkali metal hydroxide is passed through the resin to convert the resininto the hydroxide form, and to produce an effluent solution comprisinghydroxide anions and polyvalent anions. The effluent is neutralized withan acid containing .Iadd.a source of .Iaddend.polyvalent anions in orderto form a neutralized . .solution.!. .Iadd.source .Iaddend.of polyvalentanions. This neutralized solution is then passed through a second batchof resin in the monovalent anion form.

The "neutralized . .solution.!. .Iadd.source.Iaddend." of polyvalentanions does not necessarily have neutral pH (i.e., a pH of 7) subsequentto the neutralization step. In fact, it is sometimes preferred that thepolyvalent anion solution have an acid pH. Accordingly, it is to beunderstood that the term "neutralized . .solution.!..Iadd.source.Iaddend." refers to an alkali metal hydroxide solution thathas had an acid added to it, and which may ultimately have a neutral,acidic, or even a basic pH. In any event, the pH after neutralizationwill be more acidic than the pH before neutralization.

It will also be understood in connection with the foregoing andfollowing description of the invention that a resin can rarely if everbe converted so that all of its ion exchanges sites are in a particularform. Generally, the degree of conversion depends upon a number offactors including ion concentration, pH, and temperature. Thus, inreferred to the conversion of a resin to a particular form, it should beborne in mind that such conversion refers only to the majority (usuallythe vast majority) of the ion exchange sites.

The invention, together with the objects and advantages thereof, will bebest understood by reference to the following detailed description,taken together with the drawings, in which:

FIGS. 1-4 are flow diagrams illustrating a preferred embodiment of thepresent invention.

In the embodiment shown on the drawings, it has been assumed that theresin is initially in the chloride form. In regenerating the resin, itis first converted to the sulfate form in a sulfate regeneration step bytreating it with sodium sulfate, and then to the hydroxide form in ahydroxide regeneration step by treating it with sodium hydroxide. Thus,the "polyvalent anion" employed is the sulfate anion, and the "alkalimetal hydroxide" is sodium hydroxide.

Referring to FIG. 1, in the first step of the process the resin in aregeneration column 10 is treated by passing a dilute solution of sodiumsulfate from a sodium sulfate storage unit 12 through the resin in orderto convert it from the chloride form to the sulfate form. Sufficientsodium sulfate solution is employed to convert all of the ion exchangesites in the resin. Preferably the concentration of the polyvalentanions (sulfate in this instance) in the solution should be no greaterthan 0.5 normal.

As shown in FIG. 2, after the resin has been converted to the sulfateform, a solution of sodium hydroxide and sodium sulfate is deliveredfrom an effluent sodium hydroxide storage tank 14 through the resin inthe regeneration column 10 to convert it to the hydroxide form. Forreasons that will later appear, this solution comprises primarily sodiumhydroxide, with only small amounts of sodium sulfate. The hydroxide ionconcentration is not critical, and will generally be in the range ofabout 1 to 5 normal and preferably 3.5 to 5.0 normal. After the sodiumhydroxide-sodium sulfate solution has passed through the resin, theeffluent is delivered to the drain. Before being delivered to the drain,the effluent is preferably neutralized with an acid, in this instancesulfuric acid.

Referring to FIG. 3, fresh sodium hydroxide from a fresh sodiumhydroxide storage tank 16 is next delivered to the resin. The effluentfrom the regeneration column 10 will be primarily sodium hydroxide, witha small amount of sodium sulfate formed when hydroxide ions areexchanged for sulfate ions in the resin. Only a small amount of sodiumsulfate will ordinarily be formed, as the bulk of the sulfate anions isremoved in the initial sodium hydroxide treatment described above andshown in FIG. 2. In the preferred embodiment of the present invention,concentrated sodium hydroxide is employed, preferably having aconcentration in the range of about 3.5 to 5.0 normal.

Because such a sodium hydroxide solution in the range of 3.5 to 5.0normal will ordinarily be more dense than the anion exchange resin, itis preferred that this regeneration step be performed in a downflowdirection in a packed bed of the ion exchange resin. Such a procedurewill prevent flotation of the resin during regeneration.

The effluent sodium hydroxide and sodium sulfate are neutralized withsulfuric acid to form a sodium sulfate solution, as shown in FIG. 3.This sodium sulfate solution is returned to the effluent sodium sulfatestorage tank 12 for use in conversion of a second batch of resin to thesulfate form. Generally, it will be necessary to adjust theconcentration of the sodium sulfate to the proper level, i.e.,preferably 0.5 normal or less, although such adjustment need not be madeat this time.

After sufficient sodium sulfate solution has been formed for conversionof a second batch of resin to the sulfate form, the flow of effluentfrom the resin tank is diverted to the effluent sodium hydroxide storagetank 14 as shown in FIG. 4. Since this effluent is from the sodiumhydroxide delivered to the resin toward the end of the regeneration run,it will contain only very small amounts of sodium sulfate. This sodiumhydroxide solution containing small amounts of sodium sulfate is storedfor use in the initial portion of the hydroxide regeneration step forthe next batch of resin.

The regenerated resin is now removed from the regeneration column 10,and a new batch in the monovalent anion (e.g. chloride) form isdelivered. The above-described regeneration procedure is then repeatedwith this second batch of resin, and is continued for subsequent batchesof resin.

As is clear from the foregoing description, a substantial advantage ofthe method of the present invention is in its economy. That is, ratherthan being entirely delivered to the drain, the sodium hydroxide passedthrough the resin is preserved. Part of this sodium hydroxide isconverted to sodium sulfate by dilution, while the remainder is retainedfor use in the initial part of the regeneration of a subsequent batch ofresin.

As previously stated, numerous .Iadd.sources of .Iaddend.polyvalentanions may be employed in the method of the present invention..Iadd.These include ionizable substances that produce polyvalent anionsin solution, as well as those that produce monovalent anions, such asbisulfate and bicarbonate, which in turn, are ionizable to formpolyvalent anions. .Iaddend.These polyvalent anions are supplied by aninorganic compound, which should be a substance that readily ionizes indilute aqueous solutions. Examples of suitable ionized substances aresodium sulfate, sodium carbonate, potassium sulfate, potassiumcarbonate, sodium phosphate, potassium bisulfate, sodium bicarbonate,monobasic sodium phosphate, dibasic sodium phosphate, and tetrasodiumpyrophosphate. Examples of other inorganic compounds that may beemployed are magnesium sulfate, calcium sulfate, ferrous sulfate, andzinc sulfate. Any water-soluble inorganic salt that ionizes in a diluteaqueous solution to provide polyvalent anions, particularly sulfate,carbonate, or phosphate anions, and that does not form insolublesubstances with the monovalent anions displaced from the anion exchangeresin may be employed in the process. Sodium sulfate, sodium carbonate,and sodium phosphate are the preferred compounds. As previously stated,the inorganic compound is preferably employed in a dilute solution,i.e., in a concentration not exceeding 0.5 normal and preferably in therange of about 0.01 to 0.2 normal.

The amount of solution containing .Iadd.a source of .Iaddend.polyvalentanions that is employed should be sufficient to convert at least about80% of the ion exchange sites . .to the polyvalent salt form prior totreatment.!.. .Iadd.Subsequently, the resin is treated .Iaddend.with anaqueous solution of an alkali metal hydroxide to convert the resin toits hydroxide form. As a general matter, the polyvalent anions areusually supplied to the resins in an amount corresponding to about 1 to4 times the chemical equivalent capacity of the anion exchange resin.

The . .solution containing divalent.!. .Iadd.source of polyvalent.Iaddend.anions is ordinarily fed to the resin bed at a ratecorresponding to about 1 to 5 gallons of solution per square foot ofcross section of the resin bed per minute. This cross section ismeasured perpendicularly to the flow of solution. Generally, the totalamount of inorganic salt solution delivered to the resin shouldcorrespond to about 4 to 8 pounds of the inorganic compound per cubicfoot of resin. Although not indicated in the drawings, the resin isordinarily rinsed with water prior to and subsequent to its regenerationwith the alkali metal hydroxide.

The alkali metal hydroxide solution is suitably introduced at a ratecorresponding to 0.5 to 5 gallons of solution per square foot of crosssection of the resin per minute, to regenerate the anion exchange resinto its hydroxide form, after which the resin is washed with water toremove any excess alkali metal hydroxide. As previously mentioned, theconcentration of the alkali metal hydroxide should be about 1 to 5normal, and preferably about 3.5 to 5 normal. The amount of alkali metalhydroxide supplied to the resin should generally correspond to at leastabout 2 and preferably at least about 4 gram molecular equivalents ofalkali metal hydroxide per gram atomic weight of chloride ionsoriginally contained in the exhausted resin.

As previously mentioned, the method of the present invention isapplicable to a wide range of strong-base anion exchange resins of thetype having quaternary ammonium active groups. Commercially availableexamples of such a resin include Amberlite IRA-400, manufactured by Rohm& Haas Duolite A-101-D, manufactured by the Diamond Shamrock Company;and Dowex SBR, manufactured by the Dow Chemical Company.

The following example is intended to illustrate the present invention,and should not be construed as limitative, the scope of the inventionbeing determined by the appended claims.

EXAMPLE

250 cubic feet of Amberlite IRA-400 strong-base anion exchange resinwere placed in a regeneration column, and were rinsed to remove fines. A0.75% solution of sodium sulfate having a pH of about 3.6 was deliveredthrough the resin in a quantity sufficient to supply 10 pounds of sodiumsulfate per cubic foot of resin. This sodium sulfate has been formed byneutralizing and diluting with sulfuric acid the effluent obtained inthe sodium hydroxide regeneration of a previous batch of resin. Thesodium sulfate was delivered at a rate of 4 gallons per minute persquare foot of resin cross section (measured perpendicular to flow).After passing through the resin, the sodium sulfate was delivered to thedrain.

The resin was next treated with a sodium hydroxide solution that hadbeen recovered from the latter part of the regeneration of a previousbatch of resin in the manner hereinafter described. This sodiumhydroxide had a concentration of about 14% or 3.5 normal, and containedsmall quantities of sodium sulfate. The first 23% of the effluent flowwas neutralized with sulfuric acid and delivered to the drain. Theportion between 23 and 52% of the total effluent flow was neutralizedwith sulfuric acid and diluted to form a 0.75% solution of sodiumsulfate. This sodium sulfae solution was delivered to a sodium sulfatestorage tank for use in converting the next batch of resin to thesulfate form.

A total of 6250 pounds of sodium hydroxide are delivered to the resin.The first 3000 pounds were obtained from the recovering of the effluentfrom a previous regeneration. The latter 3250 pounds were introduced inthe form of a fresh 14% solution. The sodium hydroxide was introduced ata rate of 2.5 gallons per minute per square foot of surface area, anddelivered through a packed bed of the resin in a downflow direction. Theeffluent from the latter 3250 pounds of sodium hydroxide delivered tothe resin was transferred directly to an effluent sodium hydroxidestorage tank, for use in performing the initial portion of the hydroxideregeneration of a subsequent batch of resin.

One bed volume of demineralized water was next introduced into the resinat the top, and at the rate of 2.5 gallons per minute per square foot todisplace the sodium hydroxide. The initial portion of this effluent wasdelivered to the effluent sodium hydroxide storage tank, since theeffluent represents substantially pure sodium hydroxide. After abouthalf of the demineralized water had been delivered to the bed, theeffluent was neutralized and delivered to the drain.

The resin was subsequently rinsed with 75 gallons of demineralized waterper cubic foot of resin at a flow rate of 4 gallons per minute persquare foot of cross-sectional area. The rinsed, regenerated resinshowed in exceptionally high level of regeneration, with about 97% ofthe ion exchange sites in the hyroxide form. Less than 1% of the siteswere in the chloride form.

Obviously, many modifications and variations of the invention ashereinbefore set forth will occur to those skilled in the art, and it isintended to cover in the appended claims all such modifications andvariations as fall within the spirit and scope of the invention.

We claim:
 1. A method for converting strong-base anion exchange resinhaving quaternary ammonium active groups from the monovalent anion formto the hydroxide form comprising: passing a . .solution.!. .Iadd.source.Iaddend.of polyvalent anions through a first batch of said resin,whereby to displace said monovalent anions . .with said polyvalentanions.!.; subsequently passing a solution of alkali metal hydroxidethrough said resin to convert said resin to the hydroxide form, and toproduce an effluent solution of hydroxide anions and . .divalent.!..Iadd.polyvalent .Iaddend.anions; neutralizing said effluent with anacid containing .Iadd.a source of .Iaddend.polyvalent anions whereby toform a neutralized . .solution.!. .Iadd.source .Iaddend.of polyvalentanions; and passing said neutralized . .solution.!. .Iadd.source ofpolyvalent anions .Iaddend.through a second batch of said resin in themonovalent anion form.
 2. The method as defined in claim 1 wherein saidmonovalent anion form is the chloride form.
 3. The method as defined inclaim 1 wherein said polyvalent anions are divalent anions.
 4. Themethod as defined in claim 3 wherein said divalent anions are sulfateanions.
 5. The method as defined in claim 1 wherein said alkali metalhydroxide is sodium hydroxide.
 6. The method as defined in claim 5wherein said sodium hydroxide has a concentration in the range of 3.5 to5.0 normal.
 7. A method for converting strong-base anion exchange resinhaving quaternary ammonium active groups from the monovalent anion formto the hydroxide form comprising: passing a . .solution.!. .Iadd.source.Iaddend.of polyvalent anions through a first batch of said resin, saidsolution having a concentration not exceeding about 0.5 normal, todisplace said monovalent anions . .with polyvalent anions.!.;subsequently passing a solution of alkali metal hydroxide through saidresin to convert said resin to the hydroxide form and to produce aneffluent comprising an alkali metal hydroxide solution; neutralizing atleast a portion of said effluent with an acid containing .Iadd.a sourceof .Iaddend.polyvalent anions, whereby to form a neutralized effluent ..solution.!. .Iadd.source .Iaddend.of polyvalent anions; and passingsaid neutralized effluent . .solution.!. .Iadd.source of polyvalentanions .Iaddend.through a second batch of said resin in the monovalentanion form whereby to displace said monovalent anions.
 8. The method asdefined in claim 7 wherein a first portion of said effluent isneutralized, and further comprising the steps of storing a secondportion of said effluent alkali metal hydroxide solution; and passingsaid effluent alkali metal hydroxide solution through said second batchof resin after said neutralized effluent.
 9. The method as defined inclaim 8 further comprising the step of passing a fresh alkali metalhydroxide solution through said resin after said effluent alkali metalhydroxide.
 10. A method for converting strong-base anion exchange resinhaving quaternary ammonium groups from the chloride form to thehydroxide form comprising: passing a solution of sodium sulfate having aconcentration not exceeding 0.5 normal through a first batch of saidresin to displace said chloride ions with sulfate ions; subsequentlypassing a solution of sodium hydroxide through said resin in an amountsufficient to convert said resin to the hydroxide form, and to producean effluent comprising a solution of sodium hydroxide; neutralizing saideffluent with sulfuric acid, whereby to form a neutralized effluentsolution of sodium sulfate; and passing said neutralized effluentsolution through a second batch of said resin to convert said secondbatch to the sulfate form.
 11. The method as defined in claim 10 furthercomprising the steps of storing a second portion of said effluent sodiumhydroxide solution; passing said second portion of said effluent sodiumhydroxide solution through said second batch of resin after passing saidneutralized effluent through said resin; and subsequently passing afresh sodium hydroxide solution through said second batch of said resin.